(1) Field of the Invention
The present invention relates to an illuminating optical system for a projector typified by a liquid crystal projector.
(2) Description of the Related Art
A known illuminating optical system for a liquid crystal projector comprises an integrator that uniformizes the light intensity of a luminous flux from a light source (Japanese Patent Laid-Open No. 2001-228440 and Japanese Patent Laid-Open No. 2004-045907). The arrangement of optical elements of the illuminating optical system varies. In a known typical arrangement of optical elements, the optical elements are L-, U-, or S-shaped. FIGS. 1, 2, and 3 show examples of an L-shaped arrangement, a U-shaped arrangement, and an S-shaped arrangement.
An illuminating optical system shown in FIG. 1 is for a three-panel liquid crystal projector and illuminates liquid crystal panels 108a to 108c with red light, green light, and blue light, respectively. The illuminating optical system has light source 100, integrators 101a and 101b, polarization converting element 102, field lens 103, dichroic mirrors 104a and 104b, mirrors 105a to 105c, condenser lenses 106a to 106c, and relay lenses 107a and 107b. 
Light source 100 comprises a lamp typified by an ultra-high pressure mercury lamp and a reflector. Light is emitted directly by the lamp or light from the lamp is reflected by the reflector, and then a substantially parallel luminous flux is emitted from the reflector. In a direction in which the luminous flux emitted by light source 100 advances, integrators 101a and 101b, polarization converting element 102, field lens 103, and dichroic mirror 104a are sequentially arranged. Integrators 101a and 101b uniformize the light intensity of the luminous flux from light source 100. Each of integrators 101a and 101b comprise a plurality of lens cells arranged in a matrix. Polarization converting element 102 polarizes the luminous flux from integrators 101a and 101b in the same direction. Polarization converting element 102 is composed of a polarized beam splitter, a phase plate, or the like. Dichroic mirror 104a reflects B (blue) light of the luminous flux from field lens 103 while allowing the remaining R (red) light and G (green) light to pass through.
Mirror 105a is located in the advancing direction of the B light reflected by dichroic mirror 104a. Condenser lens 106a and liquid crystal panel 108a are sequentially arranged in the advancing direction of the light (B light) reflected by mirror 105a. 
Dichroic mirror 104b is arranged in the advancing direction of the R light and G light having passed through dichroic mirror 104a. Dichroic mirror 104b reflects the G light, while allowing the R light to pass through. Condenser lens 106b and liquid crystal panel 108b are sequentially arranged in the advancing direction of the light (G light) reflected by dichroic mirror 104b. Relay lens 107a and mirror 105b are sequentially arranged in the advancing direction of the light (R light) having passed through dichroic mirror 104b. Relay lens 107b and mirror 105c are sequentially arranged in the advancing direction of the light reflected by mirror 105b. Condenser lens 106c and liquid crystal panel 108c are sequentially arranged in the advancing direction of the light reflected by mirror 105c. 
In the illuminating optical system, after a luminous flux is emitted by light source 100, the light intensity of the luminous flux is uniformized by integrators 101a and 101b, and the polarizing direction of the luminous flux is uniformized by polarization converting element 102. The resultant luminous flux is incident on field lens 103. The luminous flux having passed through field lens 103 is separated into R light, G light, and B light by dichroic mirrors 104a and 104b. The R light illuminates liquid crystal panel 108c, the G light illuminates liquid crystal panel 108b, and the B light illuminates liquid crystal panel 108a. 
R image light, G image light, and B image light generated by liquid crystal panels 108a to 108c are subjected to color synthesis by cross dichroic prism 109. The resultant light is projected on a screen by projection lens 110.
An illuminating optical system with a U-shaped arrangement shown in FIG. 2 is also for a three-panel liquid crystal projector but is different from the optical system shown in FIG. 1 in that fold-back mirror 111 is located between field lens 103 and dichroic mirror 104a. A luminous flux from field lens 103 is reflected by fold-back mirror 111 at substantially 90 degrees before entering dichroic mirror 104a. The optical elements are arranged in U form.
An illuminating optical system with an S-shaped arrangement shown in FIG. 3 is also for a three-panel liquid crystal projector, but in this illuminating optical system, the arrangement of dichroic mirrors 104a and 104b, mirrors 105a to 105c, condenser lenses 106a to 106c, and relay lenses 107a and 107b is laterally opposite to that in the optical system shown in FIG. 2. In this optical system, dichroic mirror 104a reflects the R light and G light in the luminous flux from fold-back mirror 111, while allowing the remaining B light to pass through. Mirror 105a is located in the advancing direction of the B light having passed through dichroic mirror 104a. Liquid crystal panel 108a is irradiated, via condenser lens 106a, with the B light reflected by mirror 105a. Dichroic mirror 104b is located in the advancing direction of the R light and G light. Dichroic mirror 104b reflects the G light, while allowing the R light to pass through. Liquid crystal panel 108b is irradiated, via condenser lens 106b, with the G light reflected by dichroic mirror 104b. The R light passed through dichroic mirror 104b passes sequentially through relay lens 107a, mirror 105b, relay lens 107b, mirror 105c, and condenser lens 106c. Liquid crystal panel 108c is then irradiated with the R light.
The above-described illuminating optical system poses the following problems.
The polarization converting element has a plurality of polarization converting sections provided in association with the intervals among arc images of the light source projected in the vicinity of the respective optical axes of the lens cells of the integrator. Each of the polarization converting sections can uniformize the polarizing directions of luminous fluxes from the lens cells. In the polarization converting elements, part of the incident light which falls out of the effective aperture (the aperture defining the range within which polarization conversion is possible) of each polarization converting element does not contribute to polarization conversion, correspondingly reducing polarization conversion efficiency.
For an illuminating optical system that superimposes, on a liquid crystal panel surface, a luminous flux having passed through each of the lens cells of the first integrator located closer to the light source, to improve the polarization conversion efficiency, it is necessary to minimize the size of each of the arc images of the light source formed on the second integrator to reduce the quantity of light falling out of the effective aperture of the polarization converting element. However, in the illuminating optical systems having the L-, U-, and S-shaped arrangements, the field lens and the condenser lens have a long synthesizing focal distance, necessarily increasing the focal distance of the first integrator. This increases the ratio (magnification) of the size of an arc in the light source to the size of each of the arc images formed on the second integrator, resulting in an increased size of each of the arc images formed on the second integrator. In particular, in an illuminating optical system that has a U- or S-shaped arrangement including a fold-back mirror between a field lens and a condenser lens, the field lens and the condenser lens have a synthesizing focal distance longer than that in an illuminating optical system having an L-shaped arrangement. The former illuminating optical system thus has reduced polarization conversion efficiency.
Interposing a new lens between the field lens and the condenser lens enables a reduction in the synthesizing focal distance of the field lens and the condenser lens and thus in the size of each of the arc images formed on the second integrator. However, in this case, the addition of the new lens reduces transmittance and increases costs.